The invention relates to techniques for eliminating noise occurring near the bipolar zero point of a digital-to-analog converter, and more particularly to techniques for eliminating major bit transition errors at the bipolar zero point.
Eighteen bit digital-to-analog converters are being used to manufacture digital audio equipment. In digital audio equipment, audio information is stored not in the form of analog signals, but in the form of binary words which represent the polarity and amplitude of selected points of sine waves being represented in digital form. BIT1 (the most significant bit) conventionally represents the polarity of the sine wave data and BITS 2-18 represent the amplitude of the sine wave.
The most significant bit (MSB) switches at the points indicated by "A" in FIG. 2. The error produced by switching of the MSB (i.e., BIT1) is the largest switching error of a digital-to-analog converter. It is very inconvenient to have this error occur at the points labeled "A", which is the bipolar zero (BPZ) point of the digital-to-analog converter, because the error is a very large percentage of the smallest audio signals, which occur around bipolar zero. This causes discernable hissing and distortion in the audio output signal produced by the digital-to-analog converter.
In the closest prior art of which the applicant is aware, the digital sine wave data is "shifted" by adding a "1" to a certain bit, for example bit 9, of each input audio word, so that the points "A" in FIG. 2 will not occur at the bipolar zero level represented by the horizontal dotted line 5 in FIG. 2. This reduces the hissing and distortion that occur as a result of the MSB switching. However, digital shifting of the sine wave data is not a completely satisfactory solution to the problem because such shifting produces a digital "overload" if the input is at a "full scale" value, i.e. when the input consists of all "1"s. It is necessary to prevent such a digital overload from occurring, since it causes severe distortion in the analog output. This requirement prevents applying of a "full scale" input of all "1"s to the DAC. Some of the dynamic range of the digital-to-analog converter therefore is lost.